1. Field of the Invention
The present invention relates to a method of manufacturing a shoe for a compressor.
2. Description of the Related Art
A compressor, that compresses a refrigerant gas, is built into a refrigerating circuit that is used as a vehicle air conditioner or the like. For example, a known variable-displacement type swash-plate compressor has a plurality of cylinder bores 91a formed in a cylinder block 91, as shown in FIG. 10. A piston 92 is accommodated within each cylinder bore 91a so as to be able to carry out a reciprocating motion. Further, a swash plate 93 is supported by a drive shaft, not shown, such that the swash plate 93 is rotatable synchronously with the drive shaft and is tiltable with respect to the drive shaft. A pair of shoes 94 are provided, on each side of the swash plate 93, between the swash plate 93 and each piston 92. As shown in FIG. 11, the upper surface of each shoe 94 forms a part of a spherical surface as a spherical surface portion 94a, and the lower surface of the shoe forms approximately a plane surface as a plane surface portion 94b. A cylindrical portion 94c is formed in the middle between the upper portion and the lower portion via a round portion R.
In a compressor having the above structure, the swash plate 93 rotates synchronously with the drive shaft and makes an inclined movement with respect to the drive shaft, and a rotary motion of the swash plate 93 is converted into a linear reciprocating motion of the piston 92 within the cylinder bore 91a via the shoes 94, based on the rotation of the drive shaft, as shown in FIG. 10. A suction, a compression, and a discharging of a refrigerant gas are carried out at the head end of the piston 92, based on these motions. During this period, the spherical surface portion 94a of each shoe 94 slides on the surface of a spherical surface seat 92a of the piston 92, and the plane surface portion 94b of the shoe slides on the surface of the swash plate 93. Therefore, the shoe 94 is required to have high size precision and small surface roughness in order to allow smooth sliding.
Conventionally, a shoe 94 has been manufactured according to a cutting process, a forging process, and a finishing process, as follows.
 less than Cutting Process greater than 
As shown in FIG. 12, a wire 70 prepared from SUJ2 (JIS Japanese Industry Standard G4805) as a high carbon chrome bearing steel is provided first. This wire 70 is cut into pieces to obtain cut pieces 71 in a cutting process S90.
 less than Forging Process greater than 
Next, in a forging process S91, each cut piece 71 is forged with a lower die 95a and an upper die 95b, by using a forging die 95 that has a single cavity 95c to form a sphere as shown in FIG. 13. As a result, an approximately spherical steel sphere 72 having a slight flash 72a is obtained as shown in FIG. 14.
 less than Finishing Process greater than 
Then, the following finishing process S92 is carried out as shown in FIG. 12. First, in a flash removing (deburring) process S92a, a flash (a burr) is removed by sandwiching the steel sphere 72 between two rotary casting boards, not shown, and by rotating the casting boards, thereby to obtain a flash-removed ball 73.
Next, in a heat treating process S92b, a hardening and a tempering are carried out to obtain a heat-treated ball 74.
In a grinding process S92c, the heat-treated ball 74 is ground with a casting board similar to that explained above and is ground with a grindstone, thereby to obtain a ground ball 75. The hard ground ball 75 obtained in this way can also be used as a ball of a rolling bearing.
Further, the ground ball 75 is annealed in an annealing process S92d, thereby to obtain an annealed ball 76 that has a slightly lower hardness than that of the ground ball 75 and that has any internal distortion removed.
Then, in a rotary grinding process S92e, the annealed balls 76 and a slurry are put into a rotary grinder not shown, and are rotated together. As a result, the annealed balls 76 are brought into contact with each other, and are mutually ground. Gloss is added to these balls, and stains adhered to the surfaces of these balls are removed.
Further, in a cleaning process S92f, an ultrasonic cleaning is carried out to remove slight stains adhered to the surfaces. A visual inspection process S92g is carried out, and an anticorrosive is coated onto the balls in an anticorrosive processing process S92h. As a result, a raw ball 77 having a true spherical shape is obtained.
In a pressing process S92i, the raw ball 77 is pressed to obtain a material 78 formed in a shoe shape.
Further, in a heat treating process S92j, a hardening and a tempering are carried out. Then, the shoe-shaped material is ground to obtain a shoe shape and a surface coarseness within a standard, in a finish grinding process S92k. The shoe-shaped material is further cleaned in a cleaning process S92l, and is dried in a drying process S92m to finally obtain a shoe 94 for a compressor.
The conventional manufacturing method employs the flash removing process S92a. Therefore, the grinding process S92c and the rotary grinding process S92e are necessary. As the steel sphere 72 is obtained in one process of the forging process S91 by using the forging die 95 consisting of the lower die 95a and the upper die 95b, it is difficult to obtain a desired shape. Therefore, the cut piece 71 having a slightly larger volume than that of a desired shoe is obtained. This cut piece 71 has a flash (burr) 72a. As a slight gap is formed between the upper die 95b and the lower die 95a of the forging die 95, the flash 72a occurs in this gap. The obtained steel sphere 72 having the flash 72a is further subjected to the flash removing process S92a, the grinding process S92c, and the rotary grinding process S92e. Based on these processes, dispersion in the volume of the raw ball 77 is eliminated. The raw ball 77 that has approximately the same volume as that of the desired shoe 94 is pressed in the pressing process S92i. As a result, the shoe-shaped material 78 also has a constant volume, and the finally-obtained shoe 94 for a compressor has high size precision. The obtained shoe 94 has small surface roughness after the heat treating process S92j and the finish grinding process S92k. 
According to the above conventional manufacturing method, however, the shoe 94 is manufactured from the raw ball 77, after the raw ball 77 has been manufactured.
In other words, according to the conventional manufacturing method, the steel sphere 72 after the forging process S91 is further subjected to many processes including the flash removing process S92a, the heat treating process S92b, the grinding process S92c, the annealing process S92d, and the rotary grinding process S92e. The raw ball 77 is completed through the above processes. Thereafter, the raw ball 77 is again subjected to the pressing process S92i that deforms the raw ball 77 to obtain the material 78. This material 78 is then subjected to the heat treating process S92j, and the finish grinding process S92k. Therefore, an extremely large number of processes are carried out on the wire 70. Consequently, the process takes a long time, and is expensive.
The present invention has been made in the light of the above problems. It is, therefore, an object of the present invention to provide a method of manufacturing a shoe for a compressor that can shorten the manufacturing time and can reduce the manufacturing cost.
In order to achieve the above object, according to a first aspect of the present invention, there is provided a method of manufacturing a shoe for a compressor comprising: a cutting process that cuts a steel wire to obtain cut pieces; a forging process that forges each cut piece to obtain a steel sphere; and a finishing process that obtains a shoe for a compressor from the steel sphere, wherein the cutting process cuts the wire into cut pieces each having a volume approximately equivalent to that of a desired shoe, the forging process sequentially forges the cut pieces with forging dies having three or more cavities, and the finishing process obtains a shoe-shaped material from the steel sphere without a heat treatment, and carries out at least a heat treatment on the obtained material, thereby to obtain the shoe for a compressor.
According to the above aspect of the invention, in the method of manufacturing a shoe for a compressor, the cutting process cuts a wire into cut pieces each having a volume approximately equivalent to that of a desired shoe. Therefore, the steel sphere obtained in this forging process does not have a surplus portion such as a flash. Further, according to this manufacturing method, the forging process sequentially forges the cut pieces with forging dies having three or more cavities. Therefore, there occurs small distortion in the cut pieces in each step of the forging process, and there is smaller occurrence of a flash. Therefore, the conventional flash removing process becomes unnecessary.
Further, according to this manufacturing method, the finishing process does not include a heat treatment processing in the step of obtaining the shoe-shaped material from the steel sphere. Therefore, the heat treating process that has been conventionally carried out on the steel sphere becomes unnecessary. The grinding process after this heat treating process also becomes unnecessary, if this heat treatment has been conventionally carried out in the oxygen atmosphere. As it is possible to omit the conventional heat treating process and omit the subsequent grinding process, the conventional annealing process also becomes unnecessary. At least, a heat treatment is carried out to the material obtained in this way, and a shoe for a compressor is obtained as a result. Therefore, it is possible to obtain a shoe based on a small number of processes that are carried out to the wire. Facilities for the processes, that can be omitted, and consumable supplies also become unnecessary.
Therefore, according to this manufacturing method, it is possible to shorten the manufacturing time, and it is also possible to reduce the manufacturing cost. As the number of processes is decreased, it is also possible to prevent wastage of energy.
Further, according to a second aspect of the present invention, the above forging process comprises: a first process that provides a first material by forming a continuous curved surface on both end surfaces and a peripheral surface of each cut piece; a second process that provides a second material by forming the first material into a barrel-shaped second material; and a third process that forms the second material into a steel sphere having an approximately spherical shape. According to tests carried out by the inventors of the present invention, no flash occurs on the steel sphere at all.
Further, according to a third aspect of the present invention, the above first process comprises: a one-end surface forging process that provides the first material by forming a continuous curved surface on one end surface and a peripheral surface of each cut piece; and an other-end surface forging process that provides the first material by forming a continuous curved surface on the other end surface and a peripheral surface of each cut piece, wherein the one-end surface forging process and the other-end surface forging process use a cavity of the same forging die. In this case, after the one-end surface forging process has been carried out to one cut piece, this cut piece is reversed and the other-end surface forging process is carried out to this cut piece. The first process has been completed in this way. Based on this arrangement, it becomes possible to form a continuous curved surface on one end surface, the other end surface and the peripheral surface of each cut piece, by using the cavity of the same forging die. Therefore, it becomes easy to manufacture the forging die. As a result, the manufacturing cost of the forging die becomes low, and the manufacturing cost of the shoe accordingly becomes low.